Individual study: Impact of the rosette crown weevil Trichosirocalus briese on the growth and reproduction of Onopordum thistles, Langa de Duero, Burgos, Spain
Briese D.T., Thomann T. & Vitou J. (2002) Impact of the rosette crown weevil Trichosirocalus briese on the growth and reproduction of Onopordum thistles. Journal of Applied Ecology, 39, 688-698
Several European thistles of the genus Onopordum have become naturalized in Australia where they have become serious pasture weeds, leading to a project aimed at their biological control. This study describes a series of field cage experiments conducted in Spain to measure the impact of the rosette crown weevil Trichosirocalus briese on Onopordum growth and reproduction, and to determine whether this justified its release in Australia for biological control purposes.
Caged plants and weevils: An experiment was set up in 1994–95 to measure the impact of T.briese on the growth and reproductive capacity of caged Onopordum thistles. Adult T.briesei were introduced at two densities on Onopordum rosette leaves in autumn and allowing oviposition and larval development in the rosettes to occur over the following winter and spring. The growth and reproduction of the plants was monitored and compared with that of control plants, also growing in cages. Cotton thistle Onopordum acanthium was preferred as test species because it is strictly biennial and produces a flowering stem following winter vernalization. Other available species were perennials and the production of a flowering stem depends on rosette size as well as vernalization. Thus they may have flowered or persisted as rosettes, which would have added an unknown variable to the experimental design.
A more detailed experiment, in which plants were kept in individual cages, was carried out in 1995–96 in the field at the CSIRO European Laboratory Montpellier, France. Although less natural, this eliminated the possibility of adult movement between individual plants, permitting a more accurate measure of the relationship between weevil density and impact.
Field data suggested that Onopordum plants in dense patches were attacked less frequently by weevils and that larger thistle rosettes were preferred for egg-laying.
The preliminary cage experiment indicated that feeding by T.briesei could damage Onopordum spp. thistles. The second experiment using plants in individual cages showed a clear relationship between the density of the weevils and the reduction in several growth parameters and seed production. At the highest densities, larval feeding could kill Onopordum rosettes before they produced flowering stems.
Conclusions: These results indicated that T.briesei has potential as to biological control agent of Onopordum spp. thistles in Australia. Following host-specificity testing, the weevil was released in Australia 1997.
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